Electronic device for the realization of topological Josephson junctions

Abstract

The invention relates to an electronic device (100, 200) for creating topological Josephson junctions, comprising: a heterostructure including: a thin spacer layer (1) less than 10 nm thick, an encapsulation layer (3), the spacer and encapsulation layers being made of hexagonal boron nitride, hBN, a neutral graphene layer (2) between the spacer and encapsulation layers, a screening electrode (4, 6, 8) configured to screen the Coulomb interaction in the neutral graphene layer, the spacer layer (1) being deposited on the screening electrode, at least two superconducting electrodes (11, 12) electrically coupled to the neutral graphene layer, spaced from the screening electrode by at least a portion (1a) of the spacer layer, the device being configured such that,When the neutral graphene layer (2) is subjected to a magnetic field perpendicular to the plane of the layer and having an intensity at which the neutral and screened graphene transitions into an insulating state, the neutral graphene layer exhibits a helical electron transport topology phase. Figure for abbreviation: [Fig. 2],

Description

Title of the invention: Electronic device for the realization of topological Josephson junctions. Technical field

[0001] The present invention relates to an electronic device for the realization of topological Josephson junctions.

[0002] The field of the invention is, without limitation, that of quantum technologies based on hybrid semiconductor / superconducting platforms. Prior art

[0003] Quantum computers can be implemented using superconducting circuits. Quantum computers operate with two-level quantum systems, called quantum bits, or qubits, implemented using Josephson junctions and enabling the control and encoding of quantum information in electromagnetic modes. The qubits are implemented with microwave resonant circuits. These microwave circuits are generally subject to decoherence or dissipation, resulting in energy loss to the environment and thus reducing their lifetime.

[0004] Topological Josephson junctions generated using technological platforms based on semiconductors with strong spin-orbit coupling, or on topological insulators coupled to superconducting contacts, have been theoretically proposed. Josephson junctions are based on an electronic transport regime in which the spin of the electrons is fixed by the direction of their propagation.

[0005] In particular, in one-dimensional systems, such as semiconductor nanowires and two-dimensional topological insulators, electrons propagating in opposite directions therefore have opposite spins. The electron transport is then said to be helical.

[0006] Certain particular quantum states, known as Majorana states, are stable quantum states that are insensitive to decoherence mechanisms, unlike quantum bits based on superconducting circuits. Majorana states can be formed in topological superconductors and are topologically protected states.

[0007] However, in existing semiconductor-based systems, intrinsic disorder and the inevitable electronic doping of the superconducting contact are currently the major obstacles to these platforms, preventing the realization of topological Josephson junctions.

[0008] Approaches to create topological Josephson junctions, based for example on topological insulators or semiconductor nanowires with strong spin-orbit coupling, have not been effective to date.

[0009] To date, no semiconductor technology platform or hybrid Josephson junction has been able to demonstrate topological Josephson junctions. Description of the invention

[0010] One object of the present invention is to overcome at least one of these drawbacks.

[0011] It is in particular an object of the present invention to propose an electronic device enabling the realization of topological Josephson junctions, arbitrating Majorana quantum states or decoherence-insensitive parafermions.

[0012] At least one of these goals is achieved with an electronic device for realizing topological Josephson junctions, the device comprising: • a heterostructure comprising: • a thin spacer layer with a thickness of less than 10 nm, • an encapsulation layer, the spacer and encapsulation layers being made of hexagonal boron nitride (hBN), and • a layer of neutral graphene deposited between the spacer layer and the encapsulation layer, • a screening electrode configured to screen the Coulomb interaction between electrons in the neutral graphene layer, the spacer layer being deposited on the screening electrode, and • at least two electrically coupled superconducting electrodes to the neutral graphene layer and separated from the screening electrode by at least a portion of the spacing layer,

[0013] the device being configured so that, when the neutral graphene layer is subjected to a magnetic field perpendicular to the plane of the layer and having an intensity for which the neutral and screened graphene transitions into an insulating state, the neutral graphene layer exhibits a topological phase with helical electronic transport.

[0014] In this document, the expressions "undoped" and "at the charge neutrality point" may be used instead of the term "neutral" to describe the state of graphene, these descriptions being equivalent.

[0015] The electronic device according to the present invention enables the realization of topological Josephson junctions. These junctions harbor quantum states, known as Majorana or parafermions, which are states insensitive to decoherence and dissipation effects. Their particular properties of non-locality and their protection Topologically, they allow quantum information to be stored indefinitely. Majorana states can therefore be implemented to realize logical quantum bits from a single physical quantum bit.

[0016] In the device according to the invention, a topological phase of graphene is used to create a Josephson junction. This helical topological phase emerges when neutral graphene is subjected to a perpendicular magnetic field and is positioned near an electrode that screens the Coulomb interactions between the electrons of the graphene. The intensity of the magnetic field corresponds to an intensity at which the neutral, screened graphene transitions into an insulating state. This magnetic field preferably has an intensity of a few teslas. For example, it is less than 3T.

[0017] Graphene is then in a quantum Hall effect regime. In this regime, coupling graphene to superconducting contacts can generate a topological Josephson junction.

[0018] The quantum Hall effect occurs in two-dimensional electron systems subjected to a perpendicular magnetic field.

[0019] Since graphene is a two-dimensional material, it exists in the form of a crystalline monolayer. The graphene layer thus allows the production of the quantum Hall effect within it, the quantum Hall effect being necessary for the generation of the helical transport phase in neutral or undoped graphene when the latter is screened by the screening electrode.

[0020] Thus, thanks to the use of a platform based on a heterostructure according to the present invention, helical electron transport is not the result of a modification of the band structure by spin-orbit coupling, as in standard topological insulators, but relies on the physics of the quantum Hall effect. The helical phase due to the quantum Hall effect is notably more robust and insensitive to disorder and imperfections in materials than when it is based on spin-orbit coupling.

[0021] Thanks to a distance between the graphene and the screening electrode of less than 10 nm, this distance being smaller than the distance between electrons in the graphene, it is possible to screen the Coulomb interactions in the graphene, allowing the generation of the helical phase in the neutral graphene.

[0022] The device according to the present invention can thus be implemented to provide quantum states enabling in particular the realization of quantum computers based on quantum bits insensitive to decoherence.

[0023] The spacer layer and the encapsulation layer are hexagonal boron nitride (hBN) layers.

[0024] The hBN has many advantages.

[0025] hBN has a crystallographic structure whose spatial structure corresponds to that of the crystallographic structure of graphene.

[0026] hBN is also a very good insulator that does not oxidize. It has a lamellar structure and is atomically flat.

[0027] Advantageously, the superconducting electrodes are made in the thickness of the encapsulation layer, in the thickness of the graphene layer and partially in the thickness of the spacing layer.

[0028] In this way, the superconducting electrodes make proper contact with the graphene layer, the graphene layer being a monolayer. The remaining thickness of the spacer layer between the screening electrode and the superconducting electrodes is sufficient to prevent a short circuit between the electrodes.

[0029] According to one embodiment, the screening electrode may comprise a doped semiconductor layer, the device further comprising a back grid electrode on which the screening electrode is deposited.

[0030] The back grid electrode may comprise a dielectric layer and a conductive substrate.

[0031] Advantageously, the doped semiconductor layer may comprise a monolayer comprising transition metal dichalchogenides (transition metal dichalchogenides, TMDC).

[0032] These materials have the advantage of being lamellar materials which can therefore be easily integrated into van der Waals heterostructures.

[0033] The dielectric layer can be made of silicon dioxide (SiO2), hafnium dioxide (HfO2), aluminium oxide (AlOX), tungsten oxide (WO3), titanium oxide (TiO2) or hexagonal boron nitride (hBN).

[0034] According to another embodiment, the screening electrode may include an electrically conductive layer, or conductive layer.

[0035] The conductive layer may comprise a lamellar metallic material, or a metallic film.

[0036] According to yet another embodiment, the screening electrode comprises a layer with a high dielectric constant, called the dielectric layer.

[0037] By “high dielectric constant”, we understand a dielectric constant greater than about 100.

[0038] When the screening electrode is made by a layer with a high dielectric constant, the device according to the invention may further include an electrostatic electrode on which the dielectric layer is deposited.

[0039] The electrostatic electrode, or grid, allows control of the doping of graphene and makes the latter neutral. Description of the figures and methods of implementation

[0040] Other advantages and features will become apparent upon examination of the detailed description of non-limiting examples and the accompanying drawings, in which: - [Fig. 1] [Fig. 1] is a schematic representation of non-limiting examples of technological platforms that can be implemented in an electronic device according to the present invention, - [Fig.2] [Fig.2] is a schematic representation of an electronic device according to a non-limiting embodiment of the present invention, - [Fig.3] [Fig.3] is a schematic representation of an electronic device according to another non-limiting embodiment of the present invention, - [Fig. 4] [Fig. 4] shows the evolution of sublevels of the zero-index Landau level of neutral graphene as well as the electrical resistance as a function of a magnetic field applied to the device according to one embodiment, and - [Fig.5] [Fig.5] shows the evolution of Landau sublevels of neutral graphene as well as electrical resistance as a function of a magnetic field applied to the device according to another embodiment.

[0041] It is understood that the embodiments described below are by no means limiting. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0042] In particular, all the variants and all the embodiments described are combinable with each other if nothing prevents this combination from a technical point of view.

[0043] In the figures, the elements common to several figures retain the same reference.

[0044] The electronic device according to the present invention implements semiconductor-based technological platforms. These platforms comprise van der Waals heterostructures, that is, structures formed by stacks of very thin, so-called two-dimensional, layers of lamellar materials. A layer is said to be two-dimensional when it contains only a single layer of atoms.

[0045] A technological platform implemented in the present invention comprises a heterostructure including: • a thin spacing layer, • an encapsulation layer, and • a layer of neutral graphene deposited between the spacer layer and the encapsulation layer.

[0046] The spacing layer and the encapsulation layer are hexagonal boron nitride (hBN) layers.

[0047] The platform also includes a screening electrode on which the heterostructure is deposited. The screening electrode is located beneath the spacer layer so that the graphene layer and the screening electrode are separated by the spacer layer.

[0048] The spacing layer has a thickness of less than 10 nm. Its thickness is therefore less than the distance between electrons, and in particular less than the magnetic length, _ [TT of the electrons, where B is the field strength lB~^eB In the magnetic field applied to the heterostructure, e is the elementary charge and h is Planck's constant. This distance condition between the screening electrode and the graphene allows for the screening of Coulombic interactions between electrons in the graphene. Screening is the necessary condition for generating a helical electron transport phase in neutral graphene, in which electrons are transported in parallel helical channels and in which the spin of electrons propagating in one direction is opposite to that of electrons propagating in the other direction.

[0049] In the device according to the present invention, van der Waals heterostructures as described above are coupled to superconducting electrodes. These electrodes have a high critical magnetic field, that is, a field higher than the magnetic field at which neutral, screened graphene transitions into an insulating state. By applying a perpendicular magnetic field, that is, a field perpendicular to the plane of the graphene layer, it is then possible to generate topological Josephson junctions.

[0050] Majorana states, sheltered by Josephson junctions, form just before the disappearance of the helical phase when the magnetic field is increased, the disappearance being due to the opening of a gap in the excitations of the edge states in the graphene.

[0051] Parafermions, that is, fractional Majorana states, can also form in Josephson junctions, particularly when the interactions Coulombic forces in helical channels are important and induce magnetic domain walls.

[0052] Embodiments of technological platforms and electronic devices in which these platforms are implemented will be described later.

[0053] Fig. 1 is a schematic representation of non-limiting examples of technological platforms that can be implemented in an electronic device according to the present invention.

[0054] The platform according to a first example, represented in Figure 1a), comprises a heterostructure composed of a thin spacer layer 1, an encapsulation layer 3 and a neutral graphene layer 2 deposited between the spacer layer 1 and the encapsulation layer 3.

[0055] This heterostructure is deposited on a layer 4 of conductive material electric.

[0056] The spacer layer 1 has a thickness of a few nanometers. Preferably, its thickness is less than 10 nm.

[0057] In the embodiment shown in Figure 1a), the screening electrode is made by the electrically conductive layer 4.

[0058] The conductive layer 4 can be a metallic film, for example, of gold, silver, copper, palladium, platinum, tin, lead, aluminum, cobalt, etc.

[0059] The conductive layer 4 can also be made of a lamellar metallic material that is exfoliable in the form of an atomically flat layer, such as bismuth chalchogenides (Bi2Se3, Bi2Te3, Bi2TeSe2, etc.), graphite, or transition metal dichalchogenides (WSe2, MoSe2, WS2, MoS2, TaSe2, TaS2, etc.).

[0060] The conductive layer 4 also acts as an electrostatic grid, allowing control of the graphene doping and ensuring its neutrality. Indeed, encapsulated graphene may not always be neutral; it may sometimes be slightly doped. A constant grid voltage then allows the doping to be adjusted to the charge neutrality point.

[0061] The assembly comprising the heterostructure and the screening electrode 4 is deposited on a substrate 5.

[0062] The technological platform according to a second example, represented in Figure 1b), also includes a heterostructure composed of a thin spacer layer 1, an encapsulation layer 3 and a neutral graphene layer 2 deposited between the spacer layer 1 and the encapsulation layer 3.

[0063] This heterostructure is deposited on a layer 6 of material with a high dielectric constant, called the dielectric layer. The dielectric constant must be greater than approximately 100.

[0064] A metallic layer 7 is located on the rear face of the dielectric layer 6.

[0065] In the embodiment shown in Figure 1b), the screening electrode is made possible by the dielectric layer 6.

[0066] The metallic layer 7 forms an electrostatic grid and allows control of the graphene doping and makes the latter neutral.

[0067] The technological platform according to a third example, represented in Figure 1), also includes a heterostructure composed of a thin spacer layer 1, an encapsulation layer 3 and a neutral graphene layer 2 deposited between the spacer layer 1 and the encapsulation layer 3.

[0068] The platform includes an electrode whose function of screening Coulomb interactions in the graphene layer can be controlled. The controllable screening electrode 8 is formed by a layer 8 of a doped semiconductor. The doping is controlled by a back-gate electrode formed by a dielectric layer 9 and a substrate 10 of conductive material, the control being achieved via a voltage applied to the conductive substrate 10.

[0069] The semiconductor layer 8 is preferably a monolayer comprising transition metal dichalcogenides (TMDCs), which are lamellar and exfoliable materials. They can be easily integrated into van der Waals heterostructures.

[0070] The dielectric layer 9 is, for example, made of SiO2, HfO2, AlOX, WO3, TiO2 or hBN.

[0071] The conductive substrate 10 is, for example, made of heavily doped silicon.

[0072] Figures 2 and 3 are schematic representations of non-limiting examples of embodiments of an electronic device according to the present invention.

[0073] In the example shown in [Fig.2], a technological platform according to the embodiment of Figure 1a) is implemented.

[0074] In the example shown in [Fig.3], a technological platform according to the embodiment of Figure le) is implemented.

[0075] The device 100 according to the first example, shown in [Fig.2], comprises a platform according to the example as described in relation to Figure 1a) and two superconducting electrodes 11, 12. The superconducting electrodes 11, 12 form one-dimensional superconducting contacts with the graphene layer 2.

[0076] The superconducting electrodes 11, 12 are formed within the thickness of the encapsulation layer 3, within the thickness of the graphene layer 2, and partially within the thickness of the spacer layer 1. This arrangement allows contact with the graphene layer correctly, the graphene layer being a monolayer. The remaining thickness of the hBN spacer layer 1 between the screening electrode 4 and the respective superconducting electrodes 11, 12 must be sufficient to prevent a short circuit between the electrodes.

[0077] To implement the device 100 as shown in [Fig.2], a magnetic field is applied perpendicularly to the plane of the graphene layer.

[0078] An external potential is applied to the screening electrode 4 so that it also fulfills its function as an electrostatic grid.

[0079] Depending on the intensity B of the magnetic field, the resistance of the graphene evolves from a quantized value to an insulating state of the graphene, passing through a transition regime in which a Josephson junction can form, harboring Majorana states.

[0080] The operation of the device will be explained in more detail with reference to [Fig.4] below.

[0081] The device 200 according to the second example, shown in [Fig.3], comprises a platform according to the example as described in relation to Figure 1) as well as two superconducting electrodes 11, 12 as described in reference to [Fig.2].

[0082] The device 200 also includes a metal top grid electrode 13. The top grid electrode 13 allows control of the doping in the graphene layer by standard field effect.

[0083] To implement the device 200 as shown in [Fig.3], a magnetic field is applied perpendicular to the plane of the graphene layer, and a back gate voltage is applied to the conductive substrate 10.

[0084] Depending on the applied back-gate voltage, the doping of the controllable screening electrode 8 can be reduced or increased, and the electrode 8 can be made insulating or conductive. When the electrode 8 is insulating, it has no screening function. When the electrode is conductive, and depending on the doping, the screening function is present and can be adjusted. The evolution of the graphene resistance can therefore also be controlled.

[0085] The operation of the device will be explained in more detail with reference to [Fig.5] below.

[0086] Figures 4 and 5 show the evolution of the Landau sublevels of the zero-energy Landau level, called the zero-index Landau level, of the neutral graphene and of the resistance as a function of the magnetic field applied for the device according to the embodiments shown in Figures 2 and 3, respectively.

[0087] In Figure 4a) - c), we observe the evolution of the Landau sublevels of the zero energy Landau level of graphene when the graphene is screened, for a device according to the embodiment shown in [Fig.2].

[0088] Figure 4d) shows the evolution of the electrical resistance measured at two terminals, RmeaSured, of graphene as a function of the applied magnetic field at the charge neutrality point. In the diagram, the areas corresponding to the different behaviors of the Landau sublevels represented in Figures 4a) - c) are also indicated.

[0089] The Landau level contains 4 sublevels corresponding to spin and valley degeneracies.

[0090] In region (a), a spin gap opens at low magnetic fields within the screened graphene volume, with a crossover present at the edge of the layer. The crossover of Landau sublevels with opposite spins indicates the presence of counter-propagating helical states. In the example shown, the resistance is quantized at the charge neutrality point of the graphene (region (a), the theoretical value Rheiicai being indicated by a dashed line), for a magnetic field of approximately 0.1 to 2 T, indicating quantized electron transport. Electron transport corresponds to quantized resistance, or conductance. Quantized transport corresponds to the presence of a pair of edge helical channels. The superconductor-graphene-superconductor junction is then conventional.

[0091] In region (b), referred to as the transition region, at an intermediate magnetic field, a gap opens at the edges of the graphene at the level crossing (the gap being indicated by Aedge in Figure 4b)). When this gap is smaller than the superconducting gap of the superconducting electrodes 11, 12, a topological junction is generated, this junction being characterized by the presence of Majorana states. The Majorana states are more precisely located at the edges of the superconducting electrodes 11, 12.

[0092] Coulombic interactions between helical channels increase with the strength of the magnetic field. Where Coulombic interactions play a significant role, magnetic domain walls can appear in the helical channels. These walls can reverse the spin polarization, thus producing electron backscattering. Parafermions, i.e., fractional Majorana states, can therefore be formed.

[0093] The intermediate magnetic field is approximately 2 T to approximately 3 T in the example shown.

[0094] The gap that opens indicates the transition of graphene to an insulating state, in which the gap between the sublevels is completely open at the edge and throughout the volume of the graphene (indicated by Abuik). The area (c), from approximately 3 T, corresponds to the insulating state of graphene. The superconductor-graphene-superconductor junction is also insulating.

[0095] In Figure 5a) - c), we observe the evolution of the Landau sublevels of the zero energy Landau level of graphene when the graphene is screened, for a device according to the embodiment shown in [Fig.3].

[0096] Figure 5d) shows the evolution of the electrical resistance measured at two terminals, RmeaSured, of graphene as a function of the applied magnetic field at the charge neutrality point. In the diagram, the areas corresponding to the different behaviors of the Landau sublevels represented in Figures 5a) - c) are also indicated.

[0097] To implement the device according to the embodiment shown in [Fig.3], a back-gate voltage is applied to the conductive substrate 10. The controllable screening electrode 8, held to electrical ground, can be made conductive (metallic) or insulating by means of this gate voltage.

[0098] When the controllable screening electrode is metallic, it screens the Coulomb interaction between electrons in the graphene layer, which leads to the helical state (see curve (a) in Figure 5d)).

[0099] By reducing the doping in the controllable screening electrode 8 to make it insulating, it is possible to continuously decrease the effectiveness of the screen, and the resistance of the graphene goes from curve (a) to curve (b) and finally to curve (c) in Figure 5d).

[0100] For curve (b), an energy gap (the gap being indicated by Aedge in Figure 5b) opens at low magnetic fields in the screened graphene in the quantum Hall spectrum of the edge channel due to partial screening. In this state, a large plateau appears in the resistance of the graphene, as shown in curve (b). The resistance plateau before the insulating transition has an unquantized value, greater than the expected value for helical electron transport indicated by the horizontal dashed line (Rheiicai). The energy gap, and therefore this plateau, is tunable or controllable with the back gate electrode. In this gate-tunable state, the Josephson junction becomes topological and hosts Majorana bound states.

[0101] In the example shown in curve b), the magnetic field for which the plateau appears is from about 1.5 T to about 2.5 T.

[0102] For curve (c), the graphene is not screened at all, the controllable screening electrode being made insulating by the back-gate voltage. The neutral, unscreened state of graphene under a perpendicular magnetic field is insulating, and the gap between the sublevels is completely open at the edge and in the volume of the graphene (indicated by Abuik)-

[0103] To manufacture an electronic device according to the present invention, a technological platform, such as for example described with reference to Figures 2 (a)-(c), is made according to known processes.

[0104] To obtain the superconducting contacts, an etching process is performed to achieve contact between the graphene encapsulated in the hBN layers, by etching the encapsulation layer, the graphene layer, and a portion of the spacer layer. It is important not to over-etch the lower hBN layer, so as not to short-circuit it with the screening electrode located beneath the spacer layer.

[0105] Etching with precise control of the etching depth can be achieved by reactive ion etching (RIE, such as inductively coupled plasma etching, ICP) or by atomic layer etching (ALE). The etching is first tested using a test sample of hBN, previously coated by lithography with a resin. After cleaning the resin residues by oxygen plasma, the etching rate can be precisely characterized, for example, by atomic force microscopy, or with any other instrument that allows the measurement of a thickness difference, enabling measurement accuracies on the order of nanometers.

[0106] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

Claims

Demands

1. An electronic device (100, 200) for realizing topological Josephson junctions, the device (100, 200) comprising: - a heterostructure comprising: • a thin spacer layer (1) having a thickness less than 10 nm, • an encapsulation layer (3), the spacer (1) and encapsulation (3) layers being made of hexagonal boron nitride, hBN, and • a neutral graphene layer (2) deposited between the spacer layer (1) and the encapsulation layer (3), - a screening electrode (4, 6, 8) configured to screen the Coulomb interaction between electrons in the neutral graphene layer (2), the spacer layer (1) being deposited on the screening electrode (4, 6, 8), and - at least two superconducting electrodes (11, 12) electrically coupled to the neutral graphene layer (2) and spaced from the screening electrode (4, 6, 8) by at least a part (la) of the spacing layer (1),the device (100, 200) being configured such that, when the neutral graphene layer (2) is subjected to a magnetic field perpendicular to the plane of the layer and having an intensity for which the neutral and screened graphene transitions into an insulating state, the neutral graphene layer (2) exhibits a topological phase with helical electronic transport.

2. Device (100, 200) according to any one of the preceding claims, characterized in that the superconducting electrodes (11, 12) are made in the thickness of the encapsulation layer (3), in the thickness of the graphene layer (2) and partially in the thickness of the spacer layer (1).

3. Device (200) according to claim 1 or 2, characterized in that the screening electrode (8) comprises a doped semiconductor layer (8), the device (200) further comprising a back grid electrode on which the screening electrode (8) is deposited.

4. Device (200) according to the preceding claim, characterized in that the back grid electrode comprises a dielectric layer (9) and a conductive substrate (10).

5. Device (200) according to claim 3 or 4, characterized in that the doped semiconductor layer (8) comprises a monolayer comprising transition metal dichalchogenides, TMDC.

6. Device (200) according to any one of claims 3 to 5, characterized in that the dielectric layer (9) is made of SiO2, HfO2, AlOX, WO3, TiO2 or hBN.

7. Device (100) according to claim 1 or 2, characterized in that the screening electrode (4) comprises an electrically conductive layer (4).

8. Device (100) according to the preceding claim, characterized in that the electrically conductive layer (4) comprises one of - a lamellar metallic material, - a metallic film.

9. Device according to claim 1 or 2, characterized in that the screening electrode comprises a layer with a high dielectric constant (6).

10. Device according to the preceding claim, characterized in that it further comprises an electrostatic electrode (7) on which the layer with high dielectric constant (6) is deposited.

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